The invention relates to a method of determining the azimuth .alpha. of a radiowave transponder relative to a radar system which transmits a wave having a frequency F which varies linearly with time. The radar system includes a transmitting antenna and two receiving antennas, which are located at a distance d from each other. Two beat signals Fb.sub.1 and Fb.sub.2 having frequencies fb.sub.1 and fb.sub.2, respectively, are obtained by mixing the transmitted wave and the echo wave received from the transponder at each of the two receiving antennas.
The invention also relates to an apparatus for measuring the azimuth .alpha. using the above method. The apparatus forms part of a radar system which transmits a high frequency continuous wave, which is frequency modulated with a sawtooth having a constant frequency sweep .DELTA.F and a duration T, and which simultaneously receives the previously transmitted wave which is returned by a transponder. The radar system supplies a signal Fb.sub.1 of a first beat frequency fb.sub.1, obtained by mixing the transmitted signal of the instantaneous frequency F and the signal received at a first receiving antenna, and a signal Fb.sub.2 of a second beat frequency fb.sub.2, obtained by mixing the transmitted signal of the frequency F and a signal received at a second receiving antenna. The reference direction for measuring the azimuth .alpha. of the transponder is perpendicular to a line section of length d at whose ends the receiving antennas are located.
The azimuth angle .alpha. to be determined is the angle between a predetermined direction, for example a reference axis associated with the apparatus for measuring .alpha., and an axis which extends from the measuring apparatus to a target whose angular location is to be determined. Suitably, the measuring station is located on the ground, the measuring apparatus comprises a radar interrogator, and the target is an aircraft equipped with a transponder. The measuring station may alternatively be an aircraft. In practice, the angle .alpha. to be determined is suitably the angle between the mid-perpendicular plane to the receiving antennas of said radar system and the axis between the radar system and the target. On the other hand, the transponder associated with the target may be a simple passive reflector, in so far that it is isolated in the space surrounding it.
The apparatus used for carrying out the invention may, as far as the radar interrogator is concerned, for example be of the type known from French Patent Specification No. 1,557,670 corresponding to U.S. Pat. No. 3,588,899. The radar system comprises a second receiving antenna by means of which a second beat signal Fb.sub.2 of the frequency fb.sub.2 is obtained by mixing the transmitted wave and the wave received by the second receiving antenna in a second mixer. Such a radar system serves as a distance measuring apparatus and to this end it comprises a control loop which maintains the first beat signal Fb.sub.1 at a substantially constant frequency fb.sub.1 as the distance varies. This results in a variation of the duration of the transmitted sawtooth as a linear function of the distance for a constant frequency sweep .DELTA.f of the sawtooth.
It is to be noted that the invention is not limited to this type of apparatus. It equally applies to a radar system which transmits a sawtooth of constant frequency, duration and frequency sweep and which supplies two beat signals Fb.sub.1 and Fb.sub.2 obtained by mixing of the transmitted wave and the echo wave received from the transponder.
The transponder used is for example of the type described in French Patent Specification No. 2,343,258, corresponding to U.S. Pat. No. 4,151,525 in particular with reference to FIGS. 9 and 10, by means of which the azimuth of the target can be calculated at distances greater than 100 km.
By means of the two distance measuring apparatus of the type described in the French Patent Specification No. 1,557,670, having a common transmitting antenna and each having one receiving antenna, the azimuth can be determined in known manner from two distances measured by triangulation using the formula: ##EQU1## in which:
d is the (fixed) distance between the receiving antennas
R.sub.1 is the distance between the transponder and one receiving antenna
R.sub.2 is the distance between the transponder and the other receiving antenna.
The principle of determining .alpha. is described in more detail in the previously mentioned French Patent Specification No. 2,343,258.
When .alpha. is thus determined this has the drawback that at least one distance measuring apparatus is necessary (by alternately switching the control loop from one receiving antenna to the other in which case the frequencies fb.sub.1 and fb.sub.2 are equal) and that the measurement of .alpha. is not very accurate because of the length of the signal-processing chain necessary to enable the distances R.sub.1 and R.sub.2 and their difference to be determined, which leads to an accumulation of the absolute errors produced by the various signal-processing elements, the cumulative error increasing as the distance R increases.
It is also possible to determine the angle .alpha. by means of the formula: ##EQU2## c being the velocity of propagation of an electromagnetic wave.
Such a method of determining .alpha. by measuring T, fb.sub.1 and fb.sub.2 has the same drawbacks as described in the foregoing.